In LTE-Advanced (Long Term Evolution Advanced) of 3GPP (3rd Generation Partnership Project), the introduction of relay stations (hereinafter “RNs (Relay Nodes)”) is under review (see Non Patent Literature 1). The RN is one of techniques for increasing the communication speed of mobile stations (hereinafter “UEs (User Equipments)”) located at cell edges and for increasing cell areas of base stations (hereinafter “eNBs (Evolved Node Bs)”), for example. The details of the RN architecture, which is under review in 3GPP, are disclosed in Non Patent Literature 2.
The outline of an RN system based on the RN architecture disclosed in Non Patent Literature 2 will be described below. FIG. 1 is a diagram showing a network configuration example when the RN of 3GPP is used. A base station (eNB) 1 belongs to a core network (hereinafter “CN”) 4 of a mobile communication network operator. The CN 4 includes a mobility management entity (hereinafter “MME”) and a serving gateway (hereinafter “S-GW”). The eNB 1 generates an eNB cell 10 and relays traffic between a mobile station (UE) 3 and the CN 4. A relay station (RN) 2 attaches to the eNB 1 by means of a backhaul link (BL1 in the figure). The UE 3 attaches to the eNB 1 or the RN 2 by means of an access link (AL1 or AL2 in the figure). The RN 2 attaches to the CN 4 through the backhaul link (BL1) with the eNB 1. The RN 2 generates an RN cell 20 and relays traffic between the UE 3 and the CN 4. An OAM server 5 manages the installation position of the eNB 1, and the cell information (use frequency, Physical Cell ID (PCI), and the like) of the eNB cell 10. The backhaul link and the access link will be explained later.
FIG. 2 is a diagram showing a sequence when an RN attaches to an eNB and a CN and starts an RN cell operation. The RN establishes a network connection in an operation (hereinafter “UE mode”) similar to that of an existing UE upon initial start-up (step S1). Specifically, the RN configures a Radio Resource Control (hereinafter “RRC”) connection with the eNB. When the process for the RN to attach to the eNB and the CN in the UE mode is completed, the RN transitions to an operation mode as a network node (hereinafter “RN mode”), and operates the RN cell. When the RN transitions to the RN mode, a method of controlling the backhaul link is also switched from the UE mode to the RN mode (step S2).
In a network introducing the RN, the eNB that supports the RN mode is referred to as “Donor eNB (hereinafter “DeNB”)”. Herein, only in the case of describing matters unique to the DeNB related to the connection with the RN, the term “DeNB” is used to be distinguished from a normal eNB. Further, herein, the UE (for example, UE 3-1 in FIG. 1) that attaches to the DeNB is referred to as “eNB-UE”. On the other hand, the UE (for example, UE 3-2 in FIG. 1) that directly attaches to the RN is referred to as “RN-UE”. Furthermore, in the case of describing matters common to the eNB-UE and the RN-UE, the term “UE” is simply used.
In a discussion about the RN in the 3GPP, there is a demand for supporting a multihop RN in the future. The multihop RN is a technique that makes it possible to connect an additional RN to an RN that attaches to a DeNB in a cascade configuration. Herein, in the case of describing the multihop, an RN that attaches to a lower layer of a DeNB with a radio interface is referred to as “upper RN”, and an RN that attaches to a lower layer of the upper RN is referred to as “lower RN” so as to distinguish the RNs from each other. Additionally, herein, radio interfaces between a DeNB and an RN and between an upper RN and a lower RN are called “backhaul links”. Meanwhile, radio interfaces between an eNB and an eNB-UE and between an RN and an RN-UE are called “access links”.
Furthermore, in the discussion about the RN in the 3GPP, it is assumed that the RN holds, as attaching configuration information, cell information of a DeNB cell to which the RN can attach (see Non Patent Literature 3). The attaching configuration information includes information for designating a donor base station, a cell, or a sector to which the RN should attach. Specifically, the attaching configuration information includes a frequency of a DeNB cell and a Physical Cell ID (PCI), for example. The RN specifies the DeNB to which the RN should attach, based on the attaching configuration information.